The invention relates to gas sensors. More particularly, the invention relates to a method of making mixed potential gas sensors. Even more particularly, the invention relates to an electrochemical sensor formed from a tape-cast electrolyte and a plurality of electrodes.
Electrochemical gas sensors have been developed for combustion control and environmental monitoring applications. These devices typically comprise two different catalytic electrodes deposited on a solid electrolyte. Multiple oxidation-reduction (redox) reactions occur between gaseous species and the electrodes, causing mixed potentials of differing magnitude to occur between the dissimilar electrodes. Sensors comprising metal and metal-oxide electrodes have been used to detect species such as carbon monoxide (CO), nitrogen oxides, such as NO and NO2, (also referred to hereinafter as “NOx”), and hydrocarbons. Although these sensors are responsive to reducing gases, their lack of stability, reproducibility, and selectivity has hindered further development and widespread use.
Mixed potential sensors have been made by embedding and sintering electrodes, such as metal wires and oxide pellets, in an electrolyte. This process requires that individual sensors be pressed by hand, and thus does not lend itself to the large-scale manufacture of sensors. Thin film manufacturing techniques have been used as well to make sensors. Although thin film techniques are easily adaptable to large-scale production, they require the use of expensive vacuum systems. Moreover, the use of thin film techniques is currently limited to the manufacture of hydrocarbon sensors.
Current methods of making mixed potential sensors are not readily adaptable to large-scale production. In addition, the sensors produced by these methods lack the stability, reproducibility, and selectivity that are needed to make them suitable for widespread use. Therefore, what is needed is a simplified method of making mixed potential sensors. What is also needed is a method of making sensors having improved thermal stability, reproducibility, and selectivity.